Process for the preparation of pure silica

ABSTRACT

A production process for producing high-purity silica from a crude silica source by means of fluosilic acid, including the steps of: (a) subjecting the crude silica souce and the fluosilicic acid to a reaction in a reaction stage, so as to produce silicon tetrafluoride and water; (b) selectively evaporating the silicon tetrafluoride with respect to at least a portion of at least one impurity derived from the crude silica source, and (c) reacting the silicon tetrafluoride with water to produce the high-purity silica, wherein the reaction stage (a) is performed at a temperature above 75° C.

This application draws priority from Israel Patent Application SerialNo. 148,376, filed Feb. 26, 2002.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method of producing pure silica froma crude silica feed source, and more particularly, to a method in whichthe silica feed source is reacted with fluosilicic acid to produce anultra-pure silicon tetra-fluoride by-product.

Amorphous silica is an important industrial product, for which severalindustrial processes have been developed. The chemical handbook “GmelinsHandbuch der anorganischen Chemie”, Vol. 21 (1928), p. 861, discloses aprocess of Liebig (1857) in which silica is autoclaved in the presenseof an alkaline solution. Notable among the numerous publications andpatents relating to silica production is U.S. Pat. No. 4,336,235 toDeabriges, which teaches a method for the production of sodium silicateinvolving the calcination of sand with sodium carbonate at 1,500° C.,followed by the dissolution of the solid mass in an autoclave, to obtaina sodium silicate solution. By treating this solution with hydrochloricacid, an amorphous silica product with a high surface area (200 m²/g) isobtained. The product is suitable as a filler for rubber and plastics.In another silica production process, fluosilicic acid (or a saltthereof) is hydrolyzed with alkali, whereby amorphous silicaprecipitates from fluoride solution. In the above-described processes,the amorphous silica product is contaminated by impurities present inthe mother liquor.

U.S. Pat. No. 6,312,656 to Blackwell et al., describes other patentsconcerning the production of silica, and particularly pure amorphoussilica, from siloxanes. It is emphasized by Blackwell et al., that someprocesses are costly, because of the use of siloxanes as raw material,but yield products that are suitable for the production of high qualityglass, especially optical glasses. The raw material in this process ispurified polyalkyloxane, from which the silica is obtained by thermaldecomposition. Another process in which silicon tetrachloride byhydrolysis is used, produces very pure silica, but this process is alsovery costly: For the production of one ton of silica, approximatelythree tons of expensive silicon tetrachloride are needed.

U.S. Pat. No. 5,853,685 to Erickson discloses a process for producinghigh purity silica from waste by-product silica and hydrogen fluoride.The high purity silica is obtained by the reaction of impure by-productwaste silica with hydrogen fluoride, preferably in the presence of wateror sulfuric acid, producing silicon tetrafluoride gas and a motherliquor. The silicon tetrafluoride is separated from the mother liquor,which retains the impurities originally contained within the impuresilica. The silicon tetrafluoride gas is contacted with high-puritywater, in a clean environment, to form a slurry of high purity silicaand high-purity hydrofluosilicic acid. A portion of the silica isfiltered from the slurry and washed, producing a high purity silicaproduct. The rest of the silica-hydrofluosilicic acid slurry ispreferably reacted with ammonia to form a slurry of ammonium fluorideand silica. The silica is separated from the ammonium fluoride andpreferably washed and calcined to remove any remaining ammoniumfluoride, leaving additional high purity silica product. The separatedammonium fluoride may be reacted with lime to produce additionalproducts for recycling back into the process.

There are several deficiencies with the art taught by U.S. Pat. No.5,853,685. The reaction with ammonia, separation of ammonium fluoride,and ammonia recovery stages are capital-intensive and energy-intensive,and introduce various safety concerns into the industrial process.

Moreover, as taught, the process requires the costly addition of bothhydrogen fluoride and lime, as well as an ammonia make-up stream. Thegenerally infeasible process economics are acknowledged by U.S. Pat. No.5,853,685 to Erickson:

-   -   Under current economic conditions, the price of hydrogen        fluoride, at approximately $550.00 per ton, may make it        economically impractical to purify silica in the above-described        manner in most situations. The cost for the raw materials alone        would be approximately $1.10 per pound. However, where low cost        hydrogen fluoride is available and fluosilicic acid can be        reused, the process of the present invention is economically        feasible, as the cost of the pure silica is approximately        $0.11/lb.

There is therefore a recognized need for, and it would be highlyadvantageous to have, a process for producing high-purity silica thatthat is more simple and economical than the processes known in the art,and overcomes the manifest deficiencies thereof. It would be of furtheradvantage to have a process that can utilize various low-grade sourcesof silica to produce the high-purity silica.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide asimple, efficient, and cost effective process for the production ofhigh-purity silica.

According to the teachings of the present invention there is provided aproduction process for producing high-purity silica from a crude silicasource by means of fluosilicic acid, including the steps of: (a)subjecting the crude silica source and the fluosilicic acid to areaction in a reaction stage of the production process, so as to producesilicon tetrafluoride and water; (b) selectively evaporating the silicontetrafluoride with respect to at least a portion of at least oneimpurity derived from the crude silica source, and (c) reacting thesilicon tetrafluoride with water to produce the high-purity silica andregenerated fluosilicic acid, wherein the reaction stage (a) isperformed at a temperature above 75° C.

According to further features in the described preferred embodiments,the reaction stage (a) is performed at a temperature above 85° C., morepreferably, above 90° C., and up to a boiling point temperature of theaqueous medium in the reaction stage.

According to still further features in the described preferredembodiments, the evaporate produced in step (b) contains water vapor andsilicon tetrafluoride, and the weight ratio of the water vapor to thesilicon tetrafluoride is greater than 1.0 to 1.

According to still further features in the described preferredembodiments, the weight ratio of the water vapor to the silicontetrafluoride is greater than 2.0 to 1.

According to still further features in the described preferredembodiments, the process further includes the step of: (d) recycling theregenerated fluosilicic acid within the production process.

According to still further features in the described preferredembodiments, the regenerated fluosilicic acid is substantiallycompletely recycled within the production process.

According to still further features in the described preferredembodiments, the process further includes the step of: (d) recycling theregenerated fluosilicic acid to step (a).

According to still further features in the described preferredembodiments, the reacting stage (c) includes cooling to a temperaturebelow 75° C.

According to still further features in the described preferredembodiments, the reacting stage includes cooling to a temperaturebetween 150 and 75° C.

According to still further features in the described preferredembodiments, the reacting stage includes cooling to a temperaturebetween 55° and 75° C.

According to still further features in the described preferredembodiments, a solid residue is produced in step (a), the processfurther including the step of: (d) separating the solid residue from aliquid phase from the reaction stage.

According to still further features in the described preferredembodiments, the process further includes a pre-treatment step of (d)pre-treating a raw silica source so as to achieve at least a partialdissolution of at least one impurity within the raw silica source,yielding thereby the crude silica source in an aqueous medium.

According to still further features in the described preferredembodiments, the partial dissolution is effected with an acid.

According to still further features in the described preferredembodiments, the pre-treatment step further includes effecting asolid/liquid separation after the dissolution, so as to obtain the crudesilica source for introducing in the reaction stage.

According to still further features in the described preferredembodiments, the water in step (c) includes water evaporated along withthe silicon tetrafluoride.

According to still further features in the described preferredembodiments, the water in step (c) consists solely of the waterevaporated along with the silicon tetrafluoride.

According to still further features in the described preferredembodiments, the silicon tetrafluoride evaporated in step (b) isabsorbed by the water in step (c).

According to still further features in the described preferredembodiments, the high-purity silica precipitated by absorption ishydrophilic, and has a surface area exceeding 80 m²/g.

According to still further features in the described preferredembodiments, the silicon tetrafluoride evaporated in step (b) isindirectly condensed along with the water in step (c).

According to still further features in the described preferredembodiments, the high-purity, indirectly condensed silica ishydrophobic, and has a surface area below 20 m²/g.

According to still further features in the described preferredembodiments, substantially all of the high-purity silica produced isderived from the crude silica source.

According to still further features in the described preferredembodiments, the process further includes the step of: (d) recycling theregenerated fluosilicic acid within the production process.

According to still further features in the described preferredembodiments, the high-purity silica obtained has a silica (SiO₂) contentof at least 99.9%, after calcination.

According to still further features in the described preferredembodiments, the high-purity silica obtained has a silica content of atleast 99.99%.

According to still further features in the described preferredembodiments, step (a) takes place in an aqueous medium containing thefluosilicic acid and at least a second mineral acid, and wherein thesecond acid has a concentration of less than 5% by weight.

According to still further features in the described preferredembodiments, the second acid has a concentration of less than 2% byweight.

According to still further features in the described preferredembodiments, the second acid is selected from the group consisting ofsulfuric acid, hydrochloric acid and phosphoric acid, and mixturestherof.

According to still further features in the described preferredembodiments, an excess of the fluosilicic acid is added to theproduction process, such that at least a portion of the higher-purity,regenerated fluosilicic acid is removed as a co-product.

According to still further features in the described preferredembodiments, the co-product is technical-grade or food-grade fluosilicicacid.

BRIEF DESCRIPTION OF THE FIGURE

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this regard, no attempt is made to show structuraldetails of the invention in more detail than is necessary for afundamental understanding of the invention, the description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

In the drawing:

FIG. 1 is a block diagram of the inventive method for processing asilica feed source with recirculated fluosilicic acid.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a method of producing pure silica by reacting acrude silica feed source with with fluosilicic acid so as to produce anultra-pure silicon tetra-fluoride intermediate.

The principles and operation of the method according to the presentinvention may be better understood with reference to the drawings andthe accompanying description.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawing. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

As described briefly hereinabove, U.S. Pat. No. 5,853,685 to Ericksonteaches a process for producing high purity silica from waste by-productsilica and hydrogen fluoride. The high purity silica is obtained by thereaction of impure by-product waste silica with hydrogen fluoride,preferably in the presence of water or sulfuric acid, producing silicontetrafluoride gas and a mother liquor. The silicon tetrafluoride isseparated from the mother liquor, which retains the impuritiesoriginally contained within the impure silica. The silicon tetrafluoridegas is contacted with high-purity water, in a clean environment, to forma slurry of high purity silica and high-purity hydrofluosilicic acid. Aportion of the silica is filtered from the slurry and washed. The restof the silica-hydrofluosilicic acid slurry is reacted with ammonia toform a slurry of ammonium fluoride and silica. The silica is separatedfrom the ammonium fluoride and preferably washed and calcined to removeany remaining ammonium fluoride, leaving additional high purity silicaproduct.

The process disclosed by U.S. Pat. No. 5,853,685 requires the costlyaddition of hydrogen fluoride, which is lost in the bleed stream fromthe reactor, as well as in the calcination stage. The fluoride in thehydrogen fluoride feed may also be removed from the process as ammoniumfluoride by-product, which generally involves a significant loss on thefluoride value introduced as raw material.

The process according to the present invention obviates the need forammonia-related stages, including the ammonia reaction, ammoniumfluoride separation, and ammonia recovery stages.

More importantly, and by sharp contrast to the known art, the inventiveprocess produces high-purity silica from a crude silica source usingfluosilicic acid, an available and inexpensive material with respect tohydrogen fluoride. Initially, the fluosilicic acid and the crude silicasource are reacted to produce silicon tetrafluoride and water in anaqueous medium. The silicon tetrafluoride is selectively evaporated withrespect to the impurities (which are derived from the crude silicasource and from the fluosilicic acid) in the reaction mixture.Subsequently, the silicon tetrafluoride is reacted with water at atemperature below 75° C. to produce the high-purity silica, along withfluosilicic acid.

Reference is now made to FIG. 1, in which a method for the production ofpure silica from a crude silica feed source is schematically presented.The crude silica feed 1 is admixed with fluosilicic acid 2 a in reactionstep 100. The silica in crude silica feed 1 largely dissolves, producingsilicon tetrafluoride and water according to the stoichiometry of thefollowing equation:2H₂SiF₆+SiO₂=3SiF₄+2H₂O  (1)

The reaction temperature is controlled such that the silicontetrafluoride is evaporated from the reaction mixture. The variousimpurities in the reaction mixture remain in the solid or liquid phase,such that the evaporation allows for selective removal of the silicon,as silicon tetrafluoride, from the reaction mixture in reaction step100. The reaction conditions are further controlled such that water,which is stoichiometrically produced according to equation (1), isevaporated along with the silicon tetrafluoride.

Hence, the evaporate 3, containing exclusively, or substantiallyexclusively, silicon tetrafluoride and water, is introduced to a silicaformation stage 400. Preferably, evaporate 3 is cooled in silicaformation stage 400 to produce refined silica and food-grade fluosilicicacid according to the stoichiometry of the following equation:3SiF₄+2H₂O=2H₂SiF₆+SiO₂  (2)

The suspension containing refined silica 4 is filtered in a filtrationstage 300. The fluosilicic acid is recycled in stream 2 a to reactionstage 100. The filtered silica solids 5 are then processed in a dryingstage 600, whereby high-purity silica 9 is obtained. Drying stage 600may include drying, or calcination, or drying followed by calcination,according to various techno-economic considerations.

It follows from the combination of equations (1) and (2) thattheoretically, the fluosilicic acid is neither produced nor consumed. Itis also clear that the amorphous silica is not derived from thefluosilicic acid, but from the dissolved silica. On the practical,operational level, the fluosilicic acid is recovered almost totally andthus provides for a inexpensive process with respect to known processes.A fluosilicic acid make-up stream 12 provides fluosilicic acid at a ratesubstantially equal to the fluosilicic acid losses of the process.

The process of the present invention is appropriate for both batch andcontinuous production of high-purity silica. As used herein in thespecification and in the claims section that follows, the term“recycled” and the like, used in conjunction with fluosilicic acid,refer to reuse within the process, most typically in reaction stage 100.It will be appreciated by one skilled in the art that in batchprocesses, the fluosilicic acid is recycled to the reaction stage of anadditional or subsequent batch.

As used herein in the specification and in the claims section thatfollows, the term “substantially completely recycled” and the like, usedin conjunction with fluosilicic acid, refer to complete recycle of thefluosilicic acid, with the exception of process losses in bleed streams,calination, and the like.

As used herein in the specification and in the claims section thatfollows, the term “crude silica source” refers to a raw material orcombination of raw materials, separately or together containing siliconand oxygen, such as impure silica and silicates from natural andartificial sources. Specifically included are clays, coal ash,porcellanite, and fluosilicic acid (along with water or another sourceof oxygen).

It must be emphasized that in the art taught by U.S. Pat. No. 5,853,685to Erickson, sulfuric acid is preferably added to the first reactor tomaintain a concentration of 65 to 75% free sulfuric acid. At thisconcentration, the solubility of the hydrogen fluoride is at a minimum,and the reaction of the silicon tetrafluoride with free water to formfluosilicic acid in the first reactor is kept to a minimum. A relativelydry gas is produced, preventing the precipitation of silica prior to itsreaching the precipitator.

By sharp contrast, no sulfuric acid is requisite in the process of thepresent invention. The sulfuric acid added to the reaction stage in U.S.Pat. No. 5,853,685 leaves the process in a waste stream, with cleareconomic and ecological ramifications. Moreover, in the prior-artprocess, there is an acute economic danger of fluosilicic acidproduction in the reaction stage, being that extremely costly fluoridewill be lost along with the waste stream from the reaction stage. In theinventive process, the predominant component of the liquid phase isfluosilicic acid, which is ultimately recycled back to the reactionstage, such that there is no problem with any fluosilicic acidproduction in the reaction stage.

Moreover, whereas the prior art teaches that the production of arelatively dry silicon tetrafluoride gas is highly advantageous inpreventing the precipitation of silica prior to the silica formationstage, the inventors of the present invention have discovered that byoperating the reaction mixture with a high concentration of fluosilicicacid (preferably, at least 20% by weight, and more preferably, at least25%) and with a low concentration (0-5%) of other mineral acids such assulfuric acid, a wet silicon tetrafluoride gas is advantageouslyproduced. Precipitation of silica in the vapor path leading to silicaformation stage 400 is prevented by means of insulated and/or heatedpiping, according to standard technologies. The production of wetsilicon tetrafluoride gas also obviates (or greatly reduces) the needfor external feedwater in the silica formation stage 400. Most of thiswater is ultimately returned to the reaction stage of the prior artprocess, diluting the reaction stage and requiring more concentratedacid feeds.

Preferably, the weight ratio of water to silicon tetrafluoride inevaporate 3 is greater than 1:1, more preferably, above 2:1, and mostpreferably, above 2.7:1.

The preferred temperature for reaction stage 100 of the presentinvention is above 75° C., more preferably above 85° C., and mostpreferably between 90° C. and the boiling point. It will be appreciatedby one skilled in the art that elevated boiling temperatures arepossible, depending on the composition of the aqueous solution (e.g., ifadditional mineral acids are present). Also, higher operatingtemperatures can be obtained by operating reaction stage 100 aboveatmospheric pressure.

A bleed stream is preferably drawn from reaction stage 100. Motherliquor or mother liquor with solids can be withdrawn as a bleed streamfrom reaction stage 100 (e.g., via stream 14) without a solid/liquidseparation step. However, as shown in FIG. 1, a solid/liquid separationstep is typically used to remove unreacted and/or impurity-rich solids(as well as a small portion of the mother liquor as a bleed of theaqueous phase) from the process via stream 16, while returning the bulkof the liquid phase to the process via stream 18.

According to another preferred embodiment of the present invention,silica formation stage 400 is preferably carried out below 75° C., morepreferably between 55° C.-75° C., and most preferably at about 60° C. Asdescribed hereinabove with respect to reaction stage 100, it will beappreciated by one skilled in the art that other preferred operatingtemperatures are possible, depending on the concentration of thefluosilicic acid solution, and on the operating pressure of silicaformation stage 400.

According to another preferred embodiment of the present invention,silica formation stage 400 is operated as an indirect condenser (i.e.,indirect cooling), such that the reaction takes place with the water inevaporate 3. It has been found that the high-purity silica produced fromsuch a unit operation is hydrophobic, having a correspondingly lowsurface area of less than 20 m² per gram, and more typically, less than8-10 m² per gram.

According to another preferred embodiment of the present invention,evaporate 3 containing the silicon tetrafluoride vapor is absorbed withadditional, pure external feedwater (from stream 24) in silica formationstage 400. It has been discovered that the high-purity silica producedfrom silica formation by absorption is hydrophilic, haying acorrespondingly high surface area of over 80 m² per gram, and moretypically, over 100 m² per gram.

It has been observed that the silica obtained from the absorptionprocess characteristically contains small, round particles. The silicaobtained from condensation characteristically contains particles thatlook like broken egg-shells, particles that are considerably larger thanthe silica made by condensation.

The silica obtained by absorption dissolves more readily in alkalisolution to produce pure alkali-silicate solution, than the silicaproduced by condensation. It is also difficult to make a homogeneousslurry of the latter in water, without the addition of surface-activeagents. The silica produced by condensation is, however, more suitablefor the production of vitreous silica articles, optical glasses, etc.

A wide variety of silica-containing materials are suitable as rawmaterials for the inventive process. For example, residues from theproduction of aluminum compounds, obtained from the leaching of aluminumcontaining ores by mineral acids, is a very cheap source of suitable rawmaterial. Silica is present in various clays, coal ash, and in severalother raw materials at an amount of up to about 50% (by weight). Theprice of these raw materials is quite low, and usually is in the rangeof $10 to $30 per ton. Thus, the cost of the silica raw material isnegligible.

It is acknowledged, however, that not every type of silica feed materialis appropriate for the process of the present invention. Sand, forexample, which is obtained in nature by a process of relatively hightemperatures and thus has the constitution of quartz, is not a practicalraw material.

According to another preferred embodiment of the present invention,fluosilicic acid make-up stream 12 includes low-quality fluosilicicacid.

According to another preferred embodiment of the present invention,excess fluosilicic acid make-up stream 12 contains an excess offluosilicic acid, with respect to process losses, with the balance offluosilicic acid being removed via stream 2 c as technical-grade orfood-grade product.

EXAMPLES Example 1

The raw material for the experiment was a clay residue (from a clayinitially containing 35.5%, Al₂O₃, 9.6% Fe₂O₃, 54.0% SiO₂) from whichthe aluminum oxide was leached by mineral acids at a previous stage at70° C., whereby 83% of the aluminum oxide was dissolved.

To 30.0 g of the leached residue were added 50 ml water, and thesuspension was treated by adding 66 g of concentrated sulfuric acid byheating for about one hour at 100° C.-110° C. to dissolve the iron andaluminum oxide residues. 150 ml of water were then added, and thesuspension was filtered.

The new residue was washed and dried. To the dry residue, weighing 22.7g, 445.0 g of fluosilicic acid (24%) were added, and the suspension wasevaporated to dryness. After cooling the evaporate, the mixturecontained 414 g fluosilicic acid, (23.6% H₂SiF₆) and precipitatedsilica. The silica was filtered off, washed and dried. The pure, drysilica obtained weighed 20.6 g, corresponding to approximately 90.7% ofthe residue obtained after the dissolution of the aluminum and ferricoxide. About 2.5% of the silica was not dissolved by the fluosilicicacid.

After calcining, the product contained more than 99.99% SiO₂, on a drybasis. The impurity levels in the product are provided in Table 1.

It must be emphasized that the pre-leaching stage is optional, and thatgenerally, the process can be performed without a pre-leaching stage.

Example 2

The experiment was carried out according to Example 1. First, iron andaluminum oxide were removed by the addition of 80 g sulfuric acid. Forthe reaction with fluosilicic acid, 10% more of the acid was introduced.The slurry was heated for one hour, but was not evaporated to dryness.The yield of silica from the condensate was only 65%, significantly lessthan in Example 1, due to the reduced heating. The silica obtained washydrophobic.

After washing, filtering, drying, and calcining, the product containedbetter than 99.99% SiO₂, on a dry basis. The impurity levels in theproduct are provided in Table 1.

It should be emphasized that the bulk of the impurities present in thefinal product are entrained with the vapors from reaction stage 100, andthat a more robust design and control of the system can yield silica ofeven higher purity.

Example 3

The experiment was carried out with a sample of porcellanite, asilica-containing mineral that is known to accompany phosphate orefields. The porcellanite used in the experiment contained 56.3.% silica,of which 38% was defined as soluble in caustic soda. The othercomponents of the porcellanite sample include calcium carbonate, calciumphosphate and calcium sulfate, with low concentrations of ferric oxideand aluminum oxide.

To 25.0 g of the sample were added 50 g of water and 100 g ofconcentrated hydrochloric acid. The slurry was heated for about one hourto dissolve the carbonate and other soluble residues. The insolublefraction was filtered off and washed. The weight of the residue, afterdrying, was 16.1 g, i.e., about 36.6% of the sample dissolved. Thesilica remained in the insoluble matter.

To this residue, 378 g of fluosilicic acid (24%) were added. The slurrywas heated and evaporated to dryness. The residue left was filtered offand washed. The weight of the residue was 5.5 g before drying, and 2.58g after drying. 13.52 g of the original residue were evaporated.Accordingly, 52.8% of the silica of the porcellanite was obtained in theprocess.

The vapors were condensed, whereby 352 g of fluosilicic acid (23.5)%were obtained.

The calcined product contained better than 99.99% SiO₂, on a dry basis.The impurity levels in the product are provided in Table 1.

TABLE 1 The concentration of impurities in the silica product (ppm)Element Exp. 1 Exp. 2 Exp. 3 Al 0.80 0.80 1.50 V 0.40 0.22 0.32 Fe 0.400.22 0.32 Na 7 1.5 1.7 K 0.20 0.30 <0.20 Li 0.002 0.0015 0.0015 Co <2 <2<2 Ca 5 3 2.5 Cr 0.012 0.007 0.012 Cu 0.35 0.35 0.15 Ge 0.0027 0.00220.001 Mg 0.30 0.25 0.15 Ni 0.0015 0.010 0.008 P 0.010 0.008 0.009 Ti1.80 0.045 0.038 Zn 2.00 1.90 0.07 Zr <0.05 <0.05 <0.05 The amorphoussilica produced contained better than 99.99% SiO₂ (dry basis)

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

1. A production process for producing a solid silica product from acrude, impure silica source by means of fluosilicic acid, the processcomprising the steps of: (a) subjecting the crude silica source and thefluosilicic acid to a reaction in a first stage of the productionprocess, to produce silicon tetrafluoride and water in a liquid phase,said reaction performed at a temperature above 75° C.; (b) selectivelyevaporating said silicon tetrafluoride and water with respect to atleast a portion of at least one impurity derived from the crude silicasource, to produce silicon tetrafluoride vapor and water vapor; (c)reacting said silicon tetrafluoride vapor with a water source in asecond stage to produce the silica product and a regenerated fluosilicicacid, and (d) recycling said regenerated fluosilicic acid within theproduction process, wherein said water vapor is said water source, andwherein said silicon tetrafluoride vapor is indirectly condensed alongwith said water vapor, to produce the solid silica product and saidfluosilicic acid, wherein the solid silica product has a silica (SiO₂)content of at least 99.9%, and wherein said liquid phase from said firststage is restricted from entering said second stage.
 2. The process ofclaim 1, wherein the pure silica obtained has a silica (SiO₂) content ofat least 99.99%.
 3. The process of claim 1, wherein the high-puritysilica has a surface area below 20 m²/g.
 4. The process of claim 1,wherein the high-purity silica has a surface area below 8-10 m²/g. 5.The process of claim 1, wherein a solid residue is produced in step (a),the process further comprising the step of: (e) separating said solidresidue from said liquid phase.
 6. The process of claim 5, wherein aftersaid separating, said liquid phase is directly returned to said firststage.
 7. The process of claim 1, wherein a weight ratio of said watervapor to said silicon tetrafluoride vapor is greater than 1.0 to
 1. 8.The process of claim 1, wherein a weight ratio of said water vapor tosaid silicon tetrafluoride vapor is greater than 2.0 to
 1. 9. Theprocess of claim 1, wherein a weight ratio of said water vapor to saidsilicon tetrafluoride vapor is greater than 2.7 to
 1. 10. The process ofclaim 1, further comprising a pre-treatment step of: (e) pre-treating araw silica source so as to achieve at least a partial dissolution of atleast one impurity within said raw silica source, yielding thereby saidcrude silica source in an aqueous medium.
 11. The process of claim 10,wherein said partial dissolution is effected with an acid.
 12. Theprocess of claim 10, wherein said pre-treatment step further includeseffecting a solid/liquid separation after said dissolution, so as toobtain said crude silica source for introducing in said first stage. 13.The process of claim 1, wherein step (a) takes place in an aqueousmedium, said aqueous medium containing the fluosilicic acid and at leasta second acid, and wherein said second acid has a concentration of lessthan 5% by weight.
 14. The process of claim 13, wherein said second acidhas a concentration of less than 2% by weight.
 15. The process of claim13, wherein said second acid is selected from the group consisting ofhydrochloric acid and phosphoric acid.
 16. The process of claim 14,wherein said second acid is selected from the group consisting ofsulfuric acid, hydrochloric acid, and phosphoric acid.
 17. The processof claim 13, wherein said second acid is sulfuric acid.
 18. The processof claim 1, wherein an excess of the fluosilicic acid is added to theproduction process, such that at least a portion of said regeneratedfluosilicic acid is removed as a co-product.
 19. The process of claim18, wherein said regenerated fluosilicic acid is of a higher purity withrespect to the fluosilicic acid of step (a).
 20. A production processfor producing a solid silica product from a crude, impure silica sourceby means of fluosilicic acid, the process comprising the steps of: (a)subjecting the crude silica source and the fluosilicic acid to areaction in a first stage of the production process, to produce silicontetrafluoride and water in a liquid phase, said reaction performed at atemperature above 75° C.; (b) selectively evaporating said silicontetrafluoride and water with respect to at least a portion of at leastone impurity derived from the crude silica source, to produce silicontetrafluoride vapor and water vapor, and (c) reacting said silicontetrafluoride vapor with a water source in a second stage to produce thesilica product and a regenerated fluosilicic acid, wherein said watervapor is said water source, and wherein said silicon tetrafluoride vaporis indirectly condensed along with said water vapor, to produce thesolid silica product and said fluosilicic acid, wherein the solid silicaproduct has a silica (SiO₂) content of at least 99.9%, and wherein saidliquid phase from said first stage is restricted from entering saidsecond stage, and wherein the high-purity silica has a surface areabelow 20 m2/g.
 21. The process of claim 20, wherein the high-puritysilica has a surface area below 8-10 m²/g.
 22. The process of claim 20,wherein a weight ratio of said water vapor to said silicon tetrafluoridevapor is greater than 2.7 to 1.